Method for Online Cleaning of Air Preheaters

ABSTRACT

A method of cleaning air preheaters of the type having a rotor which passes through a stream of flue gas and a stream of intake combustion air is disclosed. A soot blower is operated in step-wise fashion to blow a soot blowing medium through passageways in the rotor. The passageways are arranged in concentric rings and move at a tangential velocity which depends on the speed of the rotor and the location of the passageway relative to the center of the rotor. In the present method the speed of the rotor is adjusted in accordance with the position of the soot blower so that every passageway moves over the soot blower at the same or substantially the same tangential velocity.

FIELD OF THE INVENTION

The present invention is directed toward a method for cleaning rotatingregenerative air heaters used in coal-fired electricity generatingplants.

BACKGROUND

Techniques for preheating of air have been known and used for many yearsin connection with boilers to improve combustion and boiler efficiency.One such preheating technique employs a Ljungstrom air preheater. Thisair preheater has two side-by-side ducts with flue gas flowing throughone duct while an inflow of combustion air is passed through the otherduct, the two gas flows being in opposite directions. A rotor ispositioned to rotate through both ducts about an axis between the twoducts, transferring heat from the flue gas to the combustion air. Airpreheaters, are normally operated at sufficiently high temperatures toinhibit condensation inside the heat exchanger of pollutants such assulfuric acid vapor present in the flue gas. For example, in typicalpublished temperature guide lines for Ljungstrom air preheaters, theoutlet flue gas temperature is maintained at least above 300 degrees F(149 degrees C), and as high as 350 degrees F (177 degrees C). At thesetemperatures, aerosol condensation of gaseous sulfuric acid and theassociated corrosive effects on the preheater are minimized.

At lower temperatures, condensate, ash or other related materials fromflue gases tend to deposit over a period of time on the heat transfersurfaces known in the industry as “baskets”. As these deposits build up,the flow paths for the air and flue gas become blocked and the heattransfer capacity is reduced. Therefore, it is common for these airpreheaters to include devices for blowing air or steam at highvelocities into the rotor to dislodge the deposits. The industry refersto these devices as sootblowers.

Sootblowers for cleaning air heaters are generally of the “retractable”style or the “swing-arm” style. Advantages and disadvantages are foundin both styles.

Swing arm style soot blowers for cleaning rotary regenerative airpreheaters employ a swing-arm mounted for rotation through a set angleor arc with one or more nozzles at the end that blow the soot blowingmedium (steam, air or water) onto the rotor as the rotor turns and asthe swing-arm rotates through the arc. The soot blower is normallymounted on the cold end of the rotor which is the outlet end for theflue gas.

These soot blowers typically employ a drive mechanism which includes aworm gear and a worm wheel or chain drive which rotates a lever throwarm. A connecting link attaches the lever throw arm to a lever attachedto the soot blower arm mounting plate. This linkage arrangement causesthe lever and the soot blower mounting plate to reciprocate back andforth through an arc. This type of mechanism is disclosed in U.S. Pat.No. 6,065,528 to Fierle et al. In these systems, the swing-arm sootblower constantly changes speed or angular velocity as it sweeps acrossthe rotor. At the beginning and end of its sweep, the velocity is zerowith the maximum velocity being at the center of the sweep. Between thecenter of the sweep and the beginning and end, the velocity isconstantly speeding up or slowing down due to the linkage arrangement.Therefore, the energy of the soot blowing medium is concentrated towardsthe two ends of the nozzle travel. This causes a more rapiddeterioration of the heat exchange elements in the rotor usually towardthe center and outside periphery of the rotor.

Retractable style sootblowers have found more common application inrecent years due to their similarity to sootblowers used in otherportions of the furnace and due to an improved ability to control theirposition in the cleaning process. Retractable sootblowers consist of twoconcentric tubes, one fixed in position and attached to a media source(air, steam, water, etc.) and the other capable of controlled movementinto and out of the boiler ducts laterally along the same axis as thefixed tube. A packing material is placed between the inside surface ofthe outer tube and the outside surface of the inner tube at a the end ofthe movable tube closest to the media supply line. The end of the innertube not connected to the cleaning media is open. The end of the outertube furthest from the cleaning media line is enclosed by a rounded capcontaining one or more outlet holes (or nozzles).

Cleaning media is introduced into the stationary tube from the mediasource line through a device called a popet valve. When the valve isopened, the cleaning media is introduced under pressure into the insideof the fixed tube allowing it to travel out the open outer end and intothe volume of the outer tube. The packing material prohibits the mediafrom exiting the assembly at the joining point and forces all of themedia to travel the length of the outer tube to exit through thecleaning nozzle at the far end of the movable tube. By controlling theinsertion depth of the movable tube, direct control of the cleaningpoint within the air heater surface if achieved.

Techniques for cleaning rotary heat exchangers have been described inthe art. For example, Schoenherr, et al (U.S. Pat. No. 2,812,923)describe an apparatus which applies a cleaning liquid through ports in asector plate above the heat exchanger and withdraws the liquid throughslots in a sector plate located below the heat exchanger. Such deviceshave proven to be adequate for air preheater cleaning until recently.

In recent years, coal-fired power plants have been forced to installtail end systems to further reduce nitrogen oxides emissions. Theseprocesses inject ammonia or urea as the reducing agent. In one exampleof these emission control systems, the ammonia, which is introduced intothe flue gas upstream of a catalytic reactor, reacts with nitrogenoxides in a catalytic reactor to form nitrogen (N₂) and water (H₂O). Asmall proportion of the ammonia which has been injected or introducedoften remains in the flue gas downstream of the catalytic reactor. Thiseffect is termed ammonia slippage. Ammonia slippage is essentially afunction of the required degree of removal of nitrogen oxide, of theactivity of the catalyst, and of the quality of mixing of the injectedammonia with the flue gas. It is also important for the flow through thereactor to be uniform by means of an even flue gas velocity at alllocations of the reactor cross section and for it to be possible for allthe catalytic converter material to be reached without obstacle.

On an industrial scale, these requirements can only be achieved to alimited extent with acceptable capital outlay. Consequently, it isinevitable that ammonia slippage may occur distributed unevenly acrossthe cross-section of the reactor. On average, ammonia slippage amountsto only a few ppm. However, at some locations, levels which are amultiple of this average may occur. This ammonia as well as sulfuricacid vapor will be contained in the flue gas entering the air preheater.

The temperature of the flue gas entering the air preheater is typicallyin the range of 600 to 750 degrees F (315 to 371 degrees C). In thattemperature range, the sulfur oxides react with the ammonia from theammonia slippage in accordance with the following equation: NH₃+H₂O+SO_(3→)NH₄HSO₄, i.e., to form ammonium bisulfate, or according to theequation: 2NH₃+SO₃+H₂O→(NH₄)₂SO₄, i.e., to form ammonium sulfate.

Inside the air preheater, the gas cools to the range of 230 to 350degrees F (110 to 177 degrees C). In this temperature range, ammoniumbisulfate, in addition to sulfuric acid, condenses as a sticky liquid onthe air preheater baskets.

Because ammonium bisulfate is so sticky, it captures ash particles andrapidly plugs the gas passages of the device. These deposits may alsocause corrosion. Hitherto, this problem has been counteracted bylimiting the ammonia slippage to less than 5 ppm, and in someinstallations even to less than 2 ppm. This entails a correspondinglyhigh outlay for the required catalytic reactor volume. Nevertheless, itis impossible to rule out the possibility of a higher level of ammoniaslippage than the mean occurring at some locations over the reactorcross section. Therefore, in some cases a relatively high—evenexcessively high—ammonia concentration may occur, so that the airpreheater is damaged in this area by the above-mentioned processes.

Soot blowers as mentioned above are seldom effective in removingammonium bisulfate deposits. The deposits typically occur in the middlesections of the rotor where soot blower energy has dissipated before thedeposit can be reached. The presence of deposits is evidenced by anincreased pressure drop across the air preheater on both the air and thegas sides. When this occurs, the power generating unit must be shut downand the air preheater washed with high-pressure water.

SUMMARY OF THE INVENTION

The present invention is directed toward a method of cleaning thosesticky deposits, including ammonium bisulfate, sulfuric acid and othermaterials, from a rotary air preheater without shutting down the steamgenerating system. Further, this method can be used to mitigate theaccumulation of deposits to the point of pluggage by changing theoperating strategy of conventional soot blowers. This method isparticularly effective when used with the type of rotary air preheatershown in FIG. 1. This type of preheater has a heat exchange rotorcontaining adjacent passageways through which air and flue gas aredirected and which is rotated about an axis. A soot blower is capable ofbeing positioned adjacent any selected depth from the outercircumference of the rotor such that a soot blowing medium can be blownfrom the soot blower into passageways at the selected depth as the rotoris rotated.

In a conventional preheater the rotor is rotated at a constant angularvelocity. In the present method, the angular velocity of the rotor ischanged during soot blowing according to the depth of the soot blowernozzle from the outer circumference of the rotor. Specifically theangular velocity is adjusted so that all of the passageways pass overthe soot blower at the same or substantially the same tangentialvelocity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the general arrangement of one typeof air preheater with which the present invention may be used.

FIG. 2 is diagram of the air preheater and soot blower arrangement ofFIG. 1 showing the motors and controller required to practice thepresent invention.

FIG. 3 is a graph showing the pressure drop (ΔP) across the airpreheater on the air side and gas side before, during and after cleaningusing high pressure water media.

FIG. 4 is a perspective view showing the general arrangement of a secondtype of air preheater with which the present invention may be used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a perspective view of a typical air preheater in which fluegas travels in a vertical, up or down direction and is intended toillustrate one type of air preheater in which the present invention isused. The present invention may be applied to horizontal, vertical (coldend on the top) and vertical inverted (cold end on the bottom) airpreheaters. FIG. 1 depicts a vertical air preheater with the cold end onthe bottom. The air preheater comprises a rotor housing 12 in which ismounted the heat exchange rotor 14. The rotor is mounted for rotation onthe shaft 16 which extends between the upper center section 18 and thelower center section 20. The rotor is divided into sectors orpassageways 22 by the diaphragm plates 24 and heat exchange baskets 26are stacked into these sectors 22. Located at the top and bottom of theair preheater and attached to the rotor housing 12 and to the top andbottom center sections 18 and 20, are the transition duct assembliesidentified as 28, 30, 32 and 34. These transition duct assemblies attachthe air preheater to the ducting for the air supply to and the flue gasfrom a steam generator or other combustion equipment. For example, theflue gas may enter the air preheater through transition duct 28,transfer the heat to the revolving rotor 14, and exit through transitionduct 30. The combustion air enters through transition duct 32, picks upthe heat from the rotor 14 and exits through transition duct 34. Thesetransition ducts are constructed to make the transition between thegenerally circular air preheater and the rectangular power plant ducts.

One problem that is encountered with air preheaters is that the flue gaswhich is flowing through the rotor often contains particulate materialand/or condensable substances which can be deposited on the heattransfer surfaces in the baskets 26. This tends to clog up the airpreheater, reduces the heat transfer efficiency and increases the demandon the induced draft fan. This problem is usually handled by providingsoot blowing devices 13, 15 which travel across the face of the rotor asit is revolving and blow steam, air or water onto the rotor and into theflow channels through the heat transfer surface to dislodge thedeposits. Typically, there are two soot blowers, one located on the top(or hot end) of the air heater and the other located at the bottom (orcold end) as shown on FIG. 1. If only one soot blower is used the sootblower is normally located at the cold end (the lower end of FIG. 1)because most of the deposits occur at the cold end (exit of the fluegas).

Typically, the air preheater rotates in the range of ¾ to 4.0revolutions per minute (RPM). When the boiler operator wants to cleanthe air heater, the soot blowers are inserted from the outer edge andslowly progress toward the center of the rotor. This procedure workswith dust accumulations but often fails to dislodge all of the stickyliquid deposits.

The current invention allows complete cleaning regardless of the natureof the deposits. In the favored application, the soot blower is fullyinserted to the middle of the rotor with the air preheater rotating atits normal speed. Then the soot blower is retracted in a stepwisefashion. Each step can be any convenient measure, but the greatestsuccess has been achieved with steps of 15 to 60 millimeters. At eachstep, the air preheater rotor RPM is adjusted such that the tangentialvelocity of the portion of the rotor which passes over the end of thesoot blower is constant. When one complete revolution of the rotor at aspecific sootblower position is complete, the sootblower steps (orindexes) to another location, stops, rotor speed is adjusted to matchthe new insertion depth and a complete rotor cycle is executed. Completecleaning at any step may be evidenced by the high pressure spray fromthe bottom penetrating all the way to the top of the air heater whenhigh pressure water is used as the cleaning media. The size of each stepshould be chosen according to visual observation or other means.

In a further advancement of this invention, a variable speed drive motor40 (shown in FIG. 2) controls rotor RPM and is programmed through theuse of a controller 48 to automatically set the rotor RPM in proportionto the soot blower insertion distance. The controls would also specify aminimum RPM (typically 0.2 to 0.5 RPM) to prevent stalling the motorthat drives the RPM. The minimum RPM can be different for each airheater, depending on whether it is rotating vertically or horizontally,the style and effectiveness of the support bearings and on the tightnessin the sealing mechanisms. The controller may also control the motors43, 44 that advance and retract the soot blowers and may control theblower motor 46.

Further, although two soot blowers (top and bottom) are normallyrequired for complete cleaning, we have shown that only the bottom sootblower is needed when applying this method. Using only the bottom sootblower saves considerable cleaning time and also causes less wear andtear on the hot end basket material. In this manner the number of dailycleans has been shown to be reduced from 4 times per day to 1 or 2 timesper day.

The benefits of the method are shown in the following example. The airpreheater at a North Carolina electric generating station had fouledwith ammonium bisulfate deposits to a point where the steam mediablowers could not maintain acceptable pressure drop (ΔP) across therotor. The boiler had to be removed from service every few months towater-wash the air preheaters. The hot end (top section) of these airpreheaters is 29-inches thick, while the cold end (bottom section) is41-inches thick. There are 10-inches of spacing above and below thesesections where the soot blowers operate.

The power station has two air preheaters of the same size andconstruction that operate under substantially the same conditions. Onepreheater was cleaned using the present method while the other preheaterwas not cleaned. The rotor in the air preheater that was cleaned wasmodified to permit changes in the rotational speed of the rotor asdirected by a controller. We programmed the controller to adjust thespeed of the rotor in accordance with the present method. A soot blowerwas positioned opposite the innermost ring of passageways or sectors inthe rotor and was activated to blow steam through the passageways asthey moved over the soot blower. At this time, each rotor was turning atnormal operating speed. The soot blower was then moved in step-wisefashion toward the outer ring of sectors or passageways. As the sootblower reached each successive depth, the speed of the rotor was slowedsuch that the angular velocity of the sectors was the same as theangular velocity of the sectors in the innermost depth when the processbegan. After all depths had been cleaned by the soot blower, the rotorspeed was increased back to the normal operating speed at which theprocess began. Throughout the process the power station continued tooperate normally. The speed of the rotor was 1.5 RPM when the processbegan and 0.33 RPM when the last ring was cleaned. The cleaning processtook between 3 and 4 hours.

During the testing period the pressure drop across the rotor in thepreheater being cleaned, which we identify as 2B, was measured by asensor on the flue gas side and a sensor on the combustion air side ofthe rotor. Sensors were similarly positioned on the rotor not beingcleaned which we identify as 2A. FIG. 3 shows the pressure drop (ΔP)across the air preheater on the air and gas sides before, during andafter cleaning using high pressure water media. Steady operation withpartially fouled air heaters is shown on the left side of this figure onFeb. 22, 2008. The air preheater was cleaned on line using this methodtwice during the period beginning on February 22 and ending on Feb. 24,2008. During cleaning the boiler output was reduced which reduced theair flow through the preheaters.

FIG. 3 is a graph reporting the pressure drop across the combustion airside and the flue gas side of the air preheaters and the total airflowthrough the preheaters during a period from February 22 through 24,2008. The legend in the lower right that identifies the location of thesensors. 2A indicates an air preheater, the air preheater that was notcleaned. 2B identifies a preheater that was cleaned. The pressure dropacross the input air side of the preheater is identified as SH PHTR AIRDP. The pressure drop across the flue gas side of the preheater isidentified as SAH GAS SIDE DP. During the cleaning periods the boilerload was reduced which lowered the total air flow through thepreheaters. Consequently, total air flow is also shown in FIG. 3. Theactual pressure drop at each of the four sensor locations on Feb. 2,2008 at 5:54 A.M. is given in the box at the upper left of FIG. 3. Thebox in the upper right in FIG. 3 reports the pressure drop values atthese same locations on Feb. 24, 2008 at 10:46 P.M. The significantinformation on the graph is not the peaks and valleys in the curve whichcorrespond to boiler load and air flow, but the difference in pressuredrop across the preheater which was cleaned the preheater which was notcleaned. Cleaning reduced the pressure drop from 4.1-inches of water to3.3 inches of water, while the pressure drop on the uncleaned airpreheater remained at 3.6-3.8 inches of water. Similarly, effectivecleaning using this invention reduced the gas-side ΔP from 9.9 to 8.0inches of water. The air preheater not cleaned maintained a ΔP of9.0-9.6 inches of water.

The resulting benefits of implementing this invention at the referencestation over the two years during which the present method was testedinclude elimination of two scheduled 36-hour outages per year for airheater washing. Inspections showed no damage to hot end surfaces.Ammonia reagent was increased to lower NO_(x), emissions because moreammonia slip could be tolerated. Air preheater outlet gas temperaturewas reduced by minimizing the need for a bypass, thereby increasingboiler efficiency and reducing CO₂ emissions.

Although the present method was tested using a preheater of the typeshown in FIG. 1 in which the gasses flow in a vertical direction, themethod could also be used in a horizontal flow preheater. Such apreheater is shown in FIG. 4. In this type of preheater 50 the rotor 54turns in a vertical plane on shaft 52 while the flue gas and the intakeair flow in a horizontal direction through transition ducts 55, 56, 57and 58. The rotor is divided into sectors 62 by the diaphragm plates 64and heat exchange baskets 66 are stacked into these sectors 62. Sootblowers 61 and 63 are provided on the flue gas side of the preheater.The rotor 64 and soot blower or soot blowers 61 and/or 63 are operatedso that all of the passageways pass over the soot blower at the same orsubstantially the same angular velocity.

We prefer that the passageways all travel at the same tangentialvelocity during cleaning. However, this may be difficult to achieve insome systems. The operator may find it easier to clean two or moreadjacent depths without changing the speed of the rotor. This can bedone and still achieve the benefits of our cleaning method. Therefore, avariance in tangential velocity of as much as fifteen percent isacceptable. If such a variance exists we would consider all thepassageways to be traveling at substantially the same angular velocity.

While it may be preferable to clean every depth of passageways in therotor, the present method does not require cleaning of every sector. Forsome installations it may be satisfactory to clean some but not allsectors in one pass of the soot blower. Then in another pass or atanother time other sectors or passageways can be cleaned. Indeed, theremay be some preheaters in which certain passageways are rarely or nevercleaned.

Although we have described and illustrated certain present preferredembodiments of our method for online cleaning of air preheaters, ourinvention is not limited thereto, but can be variously embodied withinthe scope of the following claims.

1. A method of cleaning air preheaters of the type comprised of a heatexchange rotor containing adjacent passageways of selected depths froman outer circumference of the rotor through which air and flue gas aredirected and which is rotated about an axis and a soot blower capable ofbeing positioned adjacent any selected depth such that a soot blowingmedium can be blown from the soot blower into passageways of theselected ring as the rotor is rotated, the method comprised of:positioning the soot blower adjacent passageways at a first depth;rotating the rotor at a first selected speed so that each passageway atthe first depth passes over the soot blower; blowing a soot blowingmedium through the passageways at the first depth while the rotor isbeing rotated at the first selected speed; positioning the soot bloweradjacent passageways at a second depth; rotating the rotor at a secondselected speed so that each passageway at the second depth passes overthe soot blower; blowing a soot blowing medium through the passagewaysat the second depth while the rotor is being rotated at the secondselected speed; and selecting the first speed and the second speed sothat all of the passageways pass over the soot blower at a substantiallysame tangential velocity.
 2. The method of claim 1 wherein the rotorcontains at least one additional set of passageways at another selecteddepth and further comprising: sequentially positioning the soot bloweradjacent the passageways at another selected depth; rotating the rotorat a selected speed for each additional set of passageways so that eachpassageway of that set passes over the soot blower; blowing a sootblowing medium through the passageways in that additional set while therotor is being rotated at the selected speed for that set ofpassageways; and selecting all of the speeds so that all of thepassageways pass over the soot blower at a substantially same tangentialvelocity.
 3. The method of claim 1 wherein the soot blowing medium isair, water or steam.
 4. The method of claim 1 wherein some, but not allpassageways in the rotor are blown with soot blowing medium.
 5. Themethod of claim 1 wherein the soot blower first blows soot blowingmedium through an innermost depth of passageways.
 6. The method of claim1 wherein the soot blower first blows soot blowing medium through anoutermost depth of passageways.
 7. The method of claim 1 whereinpassageways at two adjacent depths are blown with a soot blowing mediumand the rotor turns at a same rotations per minute while all passagewaysare being blown.